if youre looking at plex, ide suggest a cpu that has good AV1 capabilities. Intel has good offerings at base i5 12th+ are a good starting point. mobo is whatever you feel that fits your form factor.
changing from a mirrored raid (raid1) to a striped raid (raid5-6) is pretty impossible plug and play. As Durandle said, you'll need to offload your data and rebuild the array.